403 Forbidden You don't have permission to access /folder-name/ on this server

I was looking for an answer to my problem, but I could'nt find any answer which solves my case.

The problem is that I can't access the app folders in my var/www/ folder. When I go to localhost/ i get the message that my server is running and I have access to phpmyadmin too. But when I go to any localhost/folder-name i get the 403 error:

Forbidden

You don't have permission to access /folder-name/ on this server.

Apache/2.2.22 (Ubuntu) Server at localhost Port 80

How can I solve it? Btw I'm using Ubuntu 13.04, and thanks for your answers. :)